The present disclosure relates to a screw centrifuge including a drum having a rotor, two axial ends and a horizontal axis of rotation. The screw centrifuge also includes a screw arranged in the drum and configured to rotate relative to the drum at a speed different than a rotation speed of the drum and also includes a bearing device located at each axial end of the drum and configured to bear the drum. The screw centrifuge also includes a plurality of spring elements configured for sprung support of the drum on a frame and at least one of the plurality of spring elements is located at each axial end of the drum.
EP 0 107 470 B1 and U.S. Pat. No. 4,504,262 disclose the drums of decanters, such as complete-casing screw centrifuges, being supported in a sprung manner. The springs are in the form of helical springs which are aligned radially with respect to the axis of rotation. A sprung support is provided between the bearing housings of the bearings of the drum and a supporting ring by threaded bolts which pass through the helical springs. The supporting ring is arranged concentrically with respect to the bearing housing and is attached to the machine frame or is connected thereto. This makes it possible to select operating rotation speeds above the main resonant frequency of the system. In design terms, there need be only a relatively small clearance between the bearing housings and the supporting rings which surround them.
WO 94/07605 discloses a similar design to the documents cited above, but with only one axial end of the drum being supported in a sprung manner.
An elongated centrifuge with a device for reducing structure-borne sound transmissions is disclosed in DE 43 15 694 A1.
Bearings which are suitable for rather physically short drums and are not supporting but are designed to be suspending are disclosed in DE 26 06 589 A1, DE 31 34 633 A1 and DE 66 09 011 U.
With regard to the technological background, see also DE 26 32 586 A1, U.S. Pat. No. 2,094,058, U.S. Pat. No. 4,640,770 and DE 711 095 C.
In comparison to this prior art, the present disclosure provides for a better sprung support from the drum, or the entire rotor with the drum, for a centrifuge of a generic type. In particular, this is intended to be suitable for elongated designs in which the ratio between the length of the rotor and the diameter of the rotor is greater than 2.
The present disclosure thus relates to a screw centrifuge including a drum having a rotor, two axial ends and a horizontal axis of rotation. The screw centrifuge also includes a screw arranged in the drum and configured to rotate relative to the drum at a speed different than a rotation speed of the drum and also includes a bearing device located at each axial end of the drum and configured to bear the drum. The screw centrifuge also includes a plurality of spring elements configured for sprung support of the drum on a frame. At least one of the plurality of spring elements is located at each axial end of the drum and each of the plurality of spring elements is aligned essentially vertically.
As noted above, the spring elements are aligned vertically or essentially vertical.
The support is provided by combined spring/damping elements or spring elements and damping elements which are separate from them.
The drum or the entire rotor with the drum is supported in a sprung manner without there being any narrow gaps in the area of the sprung support between the parts which can move relative to one another. Such narrow gaps would make the system relatively difficult to manage.
In contrast to the situation with the prior art, the present disclosure makes it possible, without any problems, to operate the drum at an operation rotation speed which is considerably above the fundamental resonant frequency, or rotor natural shape, of the system.
This results in the creation of a centrifuge with a horizontal axis of rotation, which has an optimized sprung bearing of the rotor so as to produce an optimized behavior during operation.
The screw centrifuge of the present disclosure is suitable for elongated designs in which the ratio between the length of the rotor or the drum and the diameter of the rotor or the drum is preferably greater than 2, or possibly greater than 2.5, or greater than 3.
Because of the length, natural bending shapes or bending lines of the rotor are formed at specific frequencies in very long rotors. These frequencies are generally somewhat above the normal operating rotation speeds.
Natural rotor frequencies which can limit the possible operating rotation speed are shifted toward higher frequencies by decoupling the frame mass or foundation mass. This makes it possible to considerably increase the operating rotation speed.
Since, in addition to spring characteristics, the spring elements also have significant damping characteristics. Or, since damping elements are provided in addition to the supporting spring elements, this results in the capability of specific damping of the oscillatory rotor system, and this offers a number of advantages.
For example, the deflection when passing through critical rotation speeds, such as resonant rotation speeds or resonant frequencies, for example, of the rotor system in comparison to the machine frame or foundation when the screw centrifuge is being started up and shut down, is limited to very small values. This prevents the moving parts from striking the stationary parts.
The design, according to the present disclosure, means that it is possible to operate the screw centrifuge super critically at a very high rotation speed with regard to the first natural rotor frequencies. As a result, the operating rotation speed may be above the first resonant frequency of the rotor or of the rotor parts, such as the drum and screw.
Since, furthermore, only short distances have to be covered, the gaps, for example between the drum and the thread on the screw, can even be reduced in comparison to the previously proposed solutions for super critical operation, without or with little damping.
Since the gaps are reduced, this also makes it easier to seal them.
The spring elements and the damping elements have frequency-dependent, non-constant characteristics. As a result, it is possible to minimize the deflection movements, that is to say the distances through which the rotor is deflected with respect to the foundation or the machine frame at resonant frequencies.
Since the drums are filled with liquid when they are rotating during operation, this liquid can also cause the screw centrifuge to oscillate, particularly when partially filled during starting up and shutting down.
The combination of spring and damping furthermore makes it possible to ensure that the rotor is not caused to excite impermissible oscillations from the outside.
Excitations from the outside generally occur only at a relatively low amplitude.
However, they could by accident provide excitation precisely at a resonance of the system. In the case of an excessively lightly damped system, the rotor would then carry out undesirable oscillations.
The chosen positioning of the spring elements directly on the drum bearing allows, furthermore, isotropic damping in the vertical and horizontal directions which can be influenced by suitable adaptation of the damper. This can occur, in a desired manner, as well as anisotropically. Isotropic damping is advantageous.
The damping is a function of the rotation speed and movement and is designed such that a high damping level is produced even at low rotation speeds when driving through the rotor natural frequencies. A relatively low damping level is provided at the operating rotation speed above the resonant frequency. This effectively limits the deflections when driving through the natural frequency.
The damping at resonance should be at least 3%. Good results are achieved with dampings between 10% and 30%. Damping is understood to mean the conversion of the oscillation energy to a different energy form, for example heat. The energy conversion results in the amplitudes in the region of the resonant frequency being reduced. The quotation of the damping, as percentages, should be understood within the meaning of the Lehr damping measure D as meaning the following.D=d/ω0, where d (decay constant of the envelope e-function)=(k/2 m) and ω0 (natural frequency of the undamped system)=√(c/m), and c=spring constant.
In the case of the operating rotation speed, in contrast, the low damping results in low dynamic bearing forces, which makes a long bearing life possible. In this context, it is advantageous for the system to be tuned such that the resonant frequency is reached at a rotation speed which is less than 70% of the operating rotation speed, or even less than half the operating rotation speed.
In summary, according to the present disclosure, a screw centrifuge, such as one with a complete casing, can be produced such that a high operating rotation speed can be made use of.
It should also be mentioned as being advantageous that, according to the present disclosure, despite this high operating rotation speed, a screw centrifuge is created which operates relatively quietly. That is because the structure-borne sound introduced is reduced and is particularly low because the rotating system does not transmit undamped structure-borne sound directly to a housing or to a frame.
The housing of the screw centrifuge, according to the present disclosure, also has a particularly compact design.
Other aspects of the present disclosure will become apparent from the following descriptions when considered in conjunction with the accompanying drawings.